Restriction of blood circulation due to the atherosclerotic build up of plaque in arteries is the source of much morbidity and mortality. Plaque deposits in cardiac arteries can result in angina and myocardial infarction. Plaque deposits in peripheral arteries of the limbs can result in peripheral artery disease (PAD).
PAD affects about 20% of the population over 70, and in more severe forms (which afflict about 2 million people in the US) can lead to non-healing ulcers, infection, and eventually loss of limb due to amputation. Most people die within two years of such amputations.
Although many techniques, such as stenting and balloon angioplasty, have been developed to help restore circulation to plaque occluded cardiac arteries, these methods tend to be less effective for peripheral arteries. Stents, although well suited to low-mobility cardiac arteries, tend to either restenose or frequently break in peripheral limb arteries because these arteries are subjected to greater movement and mechanical stress. Balloon angioplasty, which stretches the artery walls while it compresses and redistributes plaque, tends to cause greater and typically less acceptable amount of artery wall damage when it is used with peripheral arteries. Additionally, since angioplasty simply redistributes plaque rather than actually removing plaque, in the higher mobility peripheral arteries, the redistributed plaque tends to relatively quickly distribute itself back into an unacceptable configuration again.
From the surgical perspective, one of the most ideal ways to treat arteries blocked by plaque is to remove the plaque from the inside of the artery using an atherectomy catheter. Such catheters, which come in a variety of different designs, can be introduced into the body at a convenient location and threaded inside the artery to the plaque occluded target region (which can usually be determined exactly using fluoroscopy and appropriate radio opaque contrast dyes). Once they are at the correct region, atherectomy catheters then surgically remove the occluding plaque.
Many different types of atherectomy catheter devices have been proposed, including catheters with rotating burrs (Boston Scientific Rotablator), lasers to photo-dissolve tissue (Spectrametics Laser Catheter), and cutter-balloon catheters (Guidant AtheroCath). All have certain drawbacks, however, such as difficulty in traversing through small and torturous arteries to get to the plaque occluded target zone or zones.
One of the biggest problems plaguing prior art atherectomy catheters is the problem of gracefully handing the shaved plaque remnants. Some designs, such as the Rotablator, make no attempt at all to handle the liberated plaque fragments, and instead let the fragments migrate through the circulation. This can cause many problems, because the liberated plaque remnants can be thrombogenic, and can end up causing downstream occlusions. Other catheter designs attempt to reduce this problem by capturing the plaque shavings and safely removing them from the body. Capturing the plaque shavings also makes the samples available for pathologic and medical diagnostic examination, and may give important information as to the root causes behind the plaque build-up in the first place.
More recent atherectomy catheters, such as the Fox Hollow SilverHawk articulated rotating blade atherectomy catheter, have been designed to address such issues. The SilverHawk catheter (exemplified by U.S. patent application Ser. Nos. 10/027,418; 10/288,559; 10/896,747; and others) uses a unique rotating blade, window, and hinged hollow nose design, which can be controlled to either assume a straight position or an angled (drooped) position.
To use the SilverHawk atherectomy catheter, the operator will usually first insert a guide wire to the proper location, attach the SilverHawk to the guidewire, and introduce the SilverHawk through a convenient artery port, often located near the groin region. The operator maneuvers the SilverHawk device to the appropriate region of plaque, keeping the SilverHawk moveable angle nose in a straight configuration. Once at the target zone, the operator then bends or adjusts the angle of the SilverHawk's hollow nose. The nose contacts the artery wall opposite the plaque target, exerting pressure. Through the laws of physics, this generates an opposing pressure that in turn presses or “urges” the catheter's window and cutter against the target plaque region.
The operator will then spin-up the cutter, and move the catheter across the target zone. The rotary cutter cuts a thin strip of plaque, which is directed, by the motion of the cutter and the device's geometry, into the devices' hollow nose cone. The cuttings stay in the nose cone, where they can eventually be removed from the body and analyzed.
The SilverHawk atherectomy catheter represented a significant advance in the state of the art, because it enabled substantially longer regions (often several centimeters or more) of plaque to be shaved for each pass of the catheter over a region. An additional advantage was that the catheter could be rotated; exposing the window and the rotating blade to another region, and a target region of plaque could thus be shaved multiple times, allowing precise control over the amount and geometry of the plaque reduction process.
Although the SilverHawk catheter demonstrated the utility of this type of approach, further improvements were still desirable. In particular, the available plaque storage space in the device's hollow nose cone was limited, and improvements in trimming partially attached plaque shavings were also desirable.
The one problem with such prior art designs was that whenever the nose cone filled with plaque, the catheter needed to be pulled from the body, cleaned, and then laboriously rethreaded back to the correct location in the target zone again. This tended to significantly prolong the length and effort required for many medical procedures, and thus was undesirable to both physician and patient alike. Methods to reduce this burden were thus highly desirable.
Atherectomy design engineers face some formidable design challenges, however. In order to navigate the narrow and torturous arteries, veins and other lumens of the body, such catheters must have extremely small diameters, usually on the order of 1 to 3 millimeters (3-9 French). At the same time, the devices must be flexible enough to be threaded through such arteries, yet have sections that are rigid enough to accomplish the required positioning, cutting, and plaque storage functions.
Due to these many design constraints, mechanical designs that might be relatively simple to execute with larger diameter devices become very problematic at such extremely small diameters. Additional constraints, such as the need to use biocompatible materials, the need for extremely high reliability, and the need for accommodate a wide variety of different plaque targets in different patients make the design of such devices quite challenging.